Such system as well as a receiver and a transmitter for use in such system is known e.g. from the article "Digital Sound Broadcasting to Mobile Receivers" by B. Le Floch et at, published in "IEEE Transactions on Consumer Electronics", Volume 35, number 3, August 1989.
In the abovementioned known system each carrier position within the symbols is occupied either by a data carrier or by a virtual carrier, having a window length being at least equal to one period of the lowest data carrier frequency, the virtual carriers having no signal power.
The digital data to be broadcasted are modulated on said data carriers using differential quadrature phase shift key modulation (DQPSK). The carriers are thereafter conversed via an inverse FFT (Fast Fourder Transform) processor into I (In-phase) and Q (Quadrature) time signals, followed by a quadrature modulation of these time signals on a transmission carrier. This system can be used for broadcasting digital audio signals and is therefore indicated as OFDM DAB (Orthogonal Frequency Division Multiplex Digital Audio Broadcasting) system.
In the receiver the reverse signal processing occurs: by using a local tuning oscillator followed by a quadrature demodulator the above baseband I and Q time signals are derived from the received quadrature modulated transmission carrier. After an analog to digital conversion these baseband I and Q time signals are applied to an FFT processor, subsequently followed by a differential demodulator, a circuit for sample deinterleaving and error correction, a sound decoding device and sound reproduction means.
In a typical usage mode an FFT of 512 points is used for modulating 448 data carriers, the remaining 64 carriers being virtual carriers as these virtual carriers have no signal power and therewith no transmission capacity. The data carriers occupy the middle range of the frequency raster, the virtual carriers are located in two mutually equal numbered groups adjacent to the group of said data carriers. The virtual carriers therewith occupy frequency ranges at each of both edges of the frequency raster which fall within the transition bands of the filters, used for selecting the useful signal at the receiver's side.
Each symbol is preceded by a guard interval for dealing with multipath effects. Each frame starts with a number of system symbols, including a zero symbol, which is used for a.o. frame synchronization and for determining channel properties and a phase reference symbol, hereinafter referred to as reference symbol, for initial phase reference.
In order to avoid carrier leakage, the FFT window has to be equal to an integer number of the period of the baseband signals. This means, that the frequency deviation of the local tuning oscillator in the receiver may only deviate from that of the local oscillator in transmitter over a very small distance. A typical value for a single frequency network (SFN) with a symbolperiod Ts=1250 microsec. is 25 Hz relative to 125 MHz or 0.2 p.p.m..
In the above article an AFC (Automatic Frequency Control) system is disclosed, which is based on a detection of the systematic deviation of the DQPSK signal vector from its expected value. In this way it is possible to detect and correct a maximum phase deviation of .+-.45 degrees. Because of the use of differential modulation, this corresponds to a maximum frequency deviation of .+-.1/8Ts.
In the above example of a SFN the required stability or free running frequency tolerance range of the local oscillator is then 100 Hz relative to 125 MHz or 0.8 p.p.m.
It is a first object of the invention to provide in a system for broadcasting and receiving digital data the possibility for a dynamic and accurate synchronization of the local oscillator of the receiver with that of the transmitter while allowing the free running tolerance range of the receiver's local oscillator to be much wider than that of the known receiver. Such local oscillators are relatively cheap.
It is a second object of the invention to provide a receiver with an AFC circuit for use in such last mentioned system to realize a coarse AFC tuning of a local receiver oscillator frequency to the local transmitter oscillator frequency, which is especially suited to be used in combination with a fine AFC tuning circuit for substantively extending the tracking range of such fine AFC tuning circuit and which allows the use of a relatively cheap receiver local oscillator.
It is a third object of the invention to provide a receiver with an AFC circuit for use in the last mentioned system to realize a fine AFC tuning of a local receiver oscillator frequency to the local transmitter oscillator frequency, which, with regard to that of the known receiver has a wider tracking range and allows the use of a much cheaper local receiver oscillator.
It is a fourth object of the invention to provide a transmitter which is suitable to be used in cooperation with each of the lastmentioned receivers.
In order to meet said first object a system for broadcasting and receiving digital data within time divisional multiplexed channels, grouped in frames, each frame comprising multicarrier symbols, including data symbols and system symbols, each symbol comprising a set of orthogonal frequency division multiplexed carriers at carrier positions within a frequency raster with regular carrier spacing, according to the invention is characterized in that said frames include frequency reference symbols, each carrying information to generate a peak value at at least one reference peak position within the frequency raster being separated by at least one carrier position from each of the edges of the frequency raster and by at least two carrier positions from an eventual subsequent reference peak position.
As already mentioned above each carrier position within the symbols is occupied either by a data carrier or by a virtual carrier, the virtual carriers having no signal power.